1. Technical Field
The present invention relates to a data transmission system and method which can be selectively used with a landline telephone system or with a variety of alternative telephone systems, particularly radiotelephones.
2. Background Art
In years past, mobile data transmission was difficult in that there was no public carrier capable of selectively connecting widely separated mobile users wishing to exchange data. The first mobile information transmission systems of this type became widely available to the public in the last decade, in the form of cellular radio telephone systems. These cellular telephone systems were primarily designed for voice communications, but inventors working in the field also developed data modems for use with these cellular telephone systems. The first commercially practical modem of this type is disclosed in U.S. Pat. No. 4,697,281 to O'Sullivan, assigned to the assignee of the present application. This early modem was capable of sensing the characteristics of a cellular bus, but the hardware of such modems was designed specifically for a particular model of cellular telephone, so that the same modem could not be used with a variety of different telephones.
One early attempt to solve this problem was the development of microprocessor controlled interfaces which connected to a cellular telephone bus and provided an external connector emulating a standard RJ-11 landline telephone jack. Some of these interfaces, such as the interface disclosed in U.S. Pat. No. 4,718,080 to Serrano et al., were capable of connecting to a plurality of cellular telephones and modifying their operation to use each telephone effectively. The "AB-3X" sold by Morrison and Dempsey Communications, could function with different cellular telephones if different connecting cables and internal firmware chips were installed in the device. Other cellular interfaces emulating a landline telephone jack are shown in U.S. Pat. Nos. 4,012,596, 4,658,096, 4,775,997 and 4,922,517 to West, Jr. et al. and U.S. Pat. No. 4,737,975 to Shafer.
While effective for their stated purpose, these landline emulator interfaces have a number of significant disadvantages which render them ineffective in certain desirable applications. First, these interfaces reduce connected computing devices to the "least common denominator" control functionality of an old-style mechanical telephone. Thus, although a computer and modem connected to such an interface are capable of sophisticated control functions, and the microprocessor-controlled cellular telephone is capable of performing a variety of functions and providing digital status information, a two-wire standard analog telephone interface provided between these two microprocessor systems does not permit sophisticated interaction between the connected systems. In addition, these landline jack emulation interfaces inherently include costly and power-hungry circuitry such as dial tone generators. Finally, connecting a modem to a cellular telephone using such a system does not facilitate a high level of integration and portability which is needed to make portable data transmission practical for a variety of users. Specifically, the systems involve additional cables, circuit boards, housings, and external power supplies which must be transported by the user.
U.S. Pat. No. 4,837,812 to Takahashi et al. shows a facsimile machine which works with both a wireline telephone and a radiotelephone. A variable resistor is adjusted to provide the correct output level for the radiotelephone. The Takahashi machine is automatically switched from wireline to radio transmission modes depending on the hook status of the connected telephone sets. Takahashi does not include any enabling disclosure of radiotelephone dialing controlled by the fax machine.
U.S. Pat. No. 4,868,863 to Hartley et al. shows an interface for connecting a modem to a variety of country-specific landline telephone systems and suggests that a modem might be configured to provide either digital or analog output, but does not provide a system for connecting a modem to control a radiotelephone.
With the development of additional types of alternative telephone systems, such as airplane seat telephone systems, digital cellular telephone systems, low power personal telephone systems, and others, the deficiencies of prior art interface schemes have become even more apparent, since the number of interfaces that must be purchased and transported by a person wishing to transmit data using these systems can become quite large.
U.S. Pat. application Ser. No. 07,531.762 for "System and Method for Interfacing Computers to Diverse Telephone Networks", assigned to the assignee of the present application, discloses a novel interface scheme in which a standardized modem transmits analog data signals and generic digital command signals through a single jack. When connected to a landline telephone jack, the jack produces standard landline DTMF signals and transmits tip and ring data signals. An external interface cable specifically designed to be compatible with one or more alternative telephone system telephones can also be connected between the modem jack and such telephones. When so connected, the modem uses its generic digital command language to control the telephone, and these generic commands are translated into control signals appropriate for the particular telephone bus by circuitry located in the interface cable.
This interface scheme represents a significant advance over prior art interface schemes, because one relatively simple modem can be used with virtually any type of telephone system or telephone. The interface cable of this system, which is the part of the interface hardware that is specific to the particular telephone, is lightweight, inexpensive to manufacture compared to prior art interfaces, and can be powered by either a portable computer battery or portable telephone battery without adversely effecting battery operating time. However, if a modem of this type is to be used with a large number of different alternative telephone systems such as the aforementioned systems currently under development, the use of a cable incorporating conversion circuitry and processing capacity becomes less desirable, because of the expense involved in constructing the cables. Therefore, there is a need for an interface system and method for connecting a standardized modem to a variety of different telephone systems without intermediate signal processing hardware.